The invention relates to the synchrodyning of the asymmetrical upper and lower sidebands of a modulated carrier wave, which carrier wave may be suppressed during transmission of its sidebands, to recover at baseband the in-phase and quadrature-phase components of a complex amplitude-modulating signal.
A quadrature-amplitude-modulated (QAM) carrier wave is representative of this type of signal. A QAM carrier wave is frequently employed for the transmission of a symbol stream descriptive of digital information, with the modulating values at peak excursions of the in-phase and quadrature-phase carrier wave components defining a two-dimensional symbol constellation. A problem in superheterodyne radio receivers for QAM signals is preventing the slow rotation of this symbol constellation that arises from incorrect frequency and phase of the local oscillators employed for complex synchrodyning of the QAM signal to baseband. Improper orientation of the symbol constellation not only affects the amplitudes of the in-phase and quadrature-phase components of the baseband signal recovered by the complex synchrodyne, but also undesirably introduces cross-coupling between the components which are supposed to be separated each from the other by the complex synchrodyne. If the error in orientation of the symbol constellation is a static error, the cross-coupling response prior to data slicing can be reduced by using adaptive equalization filtering, either before or after demodulation is accomplished. To suppress the cross-coupling response effectively, the equalization filtering either has to be a complex filter or has to employ over-sampling. Since the adaptation of equalizing filter parameters by data-dependent methods is done slowly over an extended period of time, dynamic errors in the orientation of the symbol constellation generally cannot be corrected for.
Errors in the size of the symbol constellation are more readily accommodated than errors in rotation. Errors in the size of the symbol constellation can be either by fast-acting automatic gain control or by xe2x80x9csoftxe2x80x9d data slicing procedures.
U. S. patent No. 3,101,448 issued Aug. 24, 1963 to J. P. Costas and titled xe2x80x9cSYNCHRONOUS DETECTOR SYSTEMxe2x80x9d describes an automatic-frequency-and-phase-control (AFPC) feedback loop for controlling the frequency and phase of the local oscillator used for synchrodyning a suppressed carrier amplitude-modulation (AM) signal to baseband. This type of feedback loop is referred to as xe2x80x9cthe Costas loopxe2x80x9d by those skilled in the art of digital communications receiver design.
I have discerned that a signal with complex amplitude-modulation can be down-converted in frequency to generate a pair of orthogonal final intermediate-frequency signals, each with respective upper and lower sidebands symmetrical about a respective final intermediate-frequency carrier. That is, each of these orthogonal final I-F signals is a double-sideband amplitude-modulation signal. The carriers of these two orthogonal DSB AM signals have the same frequency and are in quadrature phasing respective to each other. Each DSB AM final I-F signal is obtained by heterodyning two carriers together with the signal having complex amplitude-modulation, so as to superpose the down-conversion result and its image. To do this, one heterodyning carrier is lower in frequency than the AM signal with complex amplitude-modulation that it is being heterodyned with and the other heterodyning carrier is higher in frequency than the AM signal with complex amplitude-modulation that it is being heterodyned with. I have also discerned that each of these two orthogonal DSB AM final-IF signals when phase-split and subjected to complex multiplication by a complex carrier of the same frequency as the final I-F signal generates real and imaginary components of a respective complex baseband product output signal. I have determined that the imaginary component of the complex baseband product output signal obtained from each of these two orthogonal DSB AM final-IF signals can be used as the error signal in an AFPC feedback loop for a local oscillator used in the plural-step synchrodyning of the AM signal with complex amplitude-modulation to baseband. I have found that as the imaginary term of the complex baseband product output signal is reduced to zero by the AFPC feedback loop, the real component of the complex baseband product output signal provides a demodulation result for one of the two orthogonal phases of the original AM signal with complex amplitude-modulation. This demodulation result is provided without accompanying demodulation result from the other of the two orthogonal phases of the original AM signal with complex amplitude-modulation.
I have determined that the imaginary components of the complex baseband product output signals obtained from the two orthogonal DSB AM final-IF signals can be combined for use as the error signal in an AFPC feedback loop for a local oscillator used in the plural-step synchrodyning of the AM signal with complex amplitude-modulation to baseband. I have found that the imaginary terms of the complex baseband product output signal can be simultaneously reduced to zero by the AFPC feedback loop. This causes the real components of the complex baseband product output signals to provide respective demodulation results for each of the two orthogonal phases of the original AM signal with complex amplitude-modulation, which respective demodulation results are orthogonal to each other. These orthogonal demodulation results are suitable for time-division multiplexing for equalization in dual-phase digital filtering, I have observed.
The invention is embodied in a receiver for asymmetrical upper and lower sidebands of a modulated carrier wave, which carrier wave may be suppressed. A first down-converter in the receiver heterodynes the asymmetrical upper and lower sidebands of the modulated carrier wave being received with a first heterodyning signal, to generate a first down-conversion result superposed on the image thereof in a first final-intermediate-frequency signal offset from zero frequency. To do this, the first heterodyning signal essentially consists of two component frequencies, one below the lower sideband and the other above the upper sideband of the modulated carrier wave.
A second down-converter in the receiver heterodynes the asymmetrical upper and lower sidebands of the modulated carrier wave being received with a second heterodyning signal that is the Hilbert transform of the first heterodyning signal. This is done to generate a second down-conversion result superposed on the image thereof in a second final-intermediate-frequency signal offset from zero frequency. The receiver further comprises a first demodulator for demodulating the first down-conversion result to recover an in-phase baseband signal and a second demodulator for demodulating the second down-conversion result to recover a quadrature-phase baseband signal.